It is, however, possible to eliminate the aliasing noise by suitably selecting the band width of the modulated component S.sub.DC and the fundamental transfer frequency (sampling frequency) f.sub.C (=1/.tau..sub.H), but this means that the band width of the modulated component S.sub.DC must be selected to be narrow. If the band width of modulated component S.sub.DC is selected, for example, about 3.5 MH.sub.z as in the ordinary case, the number N.sub.H of image sensing cells in the horizontal scanning direction must be increased because the transfer frequency f.sub.C is selected high as the band width of modulated component S.sub.DC is widened. Thus, the above methods are not practical.
Accordingly, a solid state camera free from the above problem will be now described. In such an example, as shown in FIG. 6, three CCDs 10A, 10B and 10C are used. In this case, three CCDs 10A, 10B and 10C are relatively displaced by 1/3.tau..sub.H with one another in view of space. Thus, if the side band components derived from the CCDs 10A, 10B and 10C are taken as S.sub.ma, S.sub.mb and S.sub.mc, respectively, and the read-out timing or time relation upon reading out signals from the CCDs 10A, 10B and 10C is selected to satisfy the phase difference of 120.degree., the phase difference between the adjacent side band components S.sub.ma, S.sub.mb and S.sub.mc becomes 120.degree. as shown in FIG. 7. Thus, as shown in FIG. 8, if picked up output signals S.sub.oa, S.sub.ob and S.sub.oc from the CCDs 10A, 10B and 10C which satisfy the above time relation are supplied to an adding circuit 5, the adding circuit 5 produces a picked up signal S.sub.T in which the side band components S.sub.ma, S.sub.mb and S.sub.mc are cancelled and hence there is no aliasing error. The solid state camera shown in FIG. 8 is disclosed in the U.S. Pat. No. 3,975,760, so that detailed description will be omitted. But, in FIG. 8, 6 denotes an object to be picked up, 7 an optical system, and 8 a spectroscopic system which includes, for example, half mirrors 8a, 8b and mirrors 8c, 8d. Further, 12R, 12G and 12B designate color optical filters located at the front of the CCDs 10A, 10B and 10C, 9 a matrix (decoder) circuit which is supplied with the picked up signal S.sub.T from the adding circuit 5, and 11 an encoder which is supplied with the output signal from the matrix circuit 9 and produces a color picked up (video) signal satisfying the NTSC system to be delivered to an output terminal 11a.
If the camera is constructed as shown in FIG. 8, the aliasing noise can be eliminated and hence the deterioration of picture quality caused by the aliasing noise can be avoided.
By using plural CCDs, the number N.sub.H of image sensing cells of each CCD can be decreased.
In order to obtain a desired color picked-up signal of the NTSC system at the output terminal 11a of the camera shown in FIG. 8, it is necessary to supply the composite picked-up signal S.sub.T from the adding circuit 5 to the decoder 9 and to carry out the conversion processing of the signal without failure.
In order to obtain the picked-up signal S.sub.T itself as a color picked-up (video) signal S.sub.NTSC of the NTSC system (this system will be hereinafter called as a direct NTSC system), the following conditions (I) and (II) must be at least satisfied. ##EQU1##
The condition (I) can be satisfied by suitably selecting, for example, the spectroscopic system and demodulating system, and the condition (II) can be satisfied by selecting, for example, the frequency of the transfer signal S.sub.C, which will be fed to the horizontal shift register 1C of the CCD 10, as the frequency f.sub.S (=3.579545 MH.sub.z) of the color sub-carrier.
That is, since the input light informations corresponding to the image of the object are converted to the electric signals under such a state that they are sampled at every image sensing cell, the chrominance component in the picked-up output signal S.sub.T from the CCDs 10A, 10B and 10C is obtained as a carrier chrominance signal. Further, if the transfer frequency is selected as the color sub-carrier frequency, the carrier frequency of the carrier chrominance signal becomes the transfer frequency or color sub-carrier frequency to satisfy the above conditions (I) and (II). As a result, even if the encoder 11 is not used, the color video signal of the NTSC system can be obtained finally.
By the way, if the camera is constructed to satisfy the condition (II), the spatial arrangement of image sensing cells differs from the arrangement of image sensing cells in the reproduced state, and in the arrangement of reproduced image sensing cells the arrangement becomes different at every field and every frame. As a result, a flicker appears in a reproduced picture.
The above flicker phenomenon will be described in a case of the parallel-aligned CCD. FIG. 9A shows the spatial arrangement of image sensing cells 2 at the picking up portion of a CCD, and FIGS. 9B and 9C show the arrangements of reproduced image sensing cells 2, respectively.
The number N.sub.H of image sensing cells in the horizontal scanning direction in one horizontal scanning period T.sub.H is expressed as follows. EQU N.sub.H =f.sub.S .multidot.T.sub.H ( 6)
Therefore, the displacement of the arrangement of the reproduced cells from the spacial arrangement of the cells can be obtained by the equations (6) and (4).
That is, the cell arrangement at a certain field is sufficient to consider the arrangement of the final cell of previous line.
If an odd field at an odd frame is taken as a reference of consideration, the number of image sensing cells in N lines is given as follows. EQU N.multidot.N.sub.H =N.multidot.f.sub.S .multidot.T.sub.H ( 7)
Since the following equation (8) is established EQU f.sub.H =1/T.sub.H ( 8)
the equation (7) can be expressed as follows. ##STR1##
Thus, if the number N is an odd number or since the first line of this field is N=1, the equation (9) can be rewritten as follows. ##STR2## where l is an integer.
In general, if the reading order which corresponds to a television scanning is taken into consideration, the final image sensing cell and the first cell in the following (N+1) line are arranged apart from each other by .tau..sub.H in view of space similar to the other cell arrangements. Therefore, 1/2 in the equation (10) means that the first cell in the next (second) line is displaced from the reference time of the horizontal scanning period T.sub.H by 1/2.tau..sub.H. That is, the cells between the N and N+1 lines are relatively displaced by 1/2.tau..sub.H.
Accordingly, at the odd field in the odd frame, a movement or displacement of 1/2.tau..sub.H of reproduced cells appears between the N line (odd line) and N+1 line (even line) as shown in FIG. 9B.
Next, an even field in an odd frame is now considered. In this case, since 264th line becomes the first line, the number of image sensing cells between the lines 263 and 264 can be calculated similar to the equation (9), as follows. ##EQU2## where m is an integer. Thus, the reproduced image sensing cells move by 1/2.tau..sub.H.
In the case of the even field, different from the odd field, the reproduced image sensing cells of only the odd line move, which is shown in FIG. 9B by dotted lines.
In the case of an even frame, the reproduced image sensing cells opposite to those of the odd frame move on the respective fields, which is shown in FIG. 9C.
That is, in the even frame, the reproduced cells on the odd lines of the odd field move, while the reproduced cells on the even lines of the even field move.
As may be apparent from the comparison of FIGS. 9B and 9C, the movement of the reproduced cells occur between the odd and even frames and there is a period of every two frame.
When the arrangement of the reproduced cells is moved at every field and every frame as described above, there are caused flickers and jitters and hence a reproduced picture becomes discomfortable for a viewer.
When a checkered-pattern of a CCD is used as the CCD of the solid state camera, the similar phenomenon will be caused (refer to FIGS. 10A, 10B and 10C). In this case, however, the movement of the reproduced image sensing cells appears in only one field in either of the odd and even frames.